Circuits and methods for a memory cell with a trench plate trench capacitor and a vertical bipolar read device

ABSTRACT

A memory device is described which has an n-channel field effect transistor coupled between a memory cell and a data communication line. An NPN bipolar junction transistor is also coupled between the memory cell and the data communication line in parallel to the n-channel access transistor. A base connection of the NPN bipolar junction transistor is described as coupled to a body of the n-channel access transistor. During operation the n-channel field effect transistor is used for writing data to a memory cell, while the NPN bipolar junction transistor is used for read operations in conjunction with a current sense amplifier circuit. The access transistors are described as fabricated as a single vertical pillar.

RELATED APPLICATIONS

This application is a diviosnal of U.S. Ser. No. 09/498,433 filed Aug.4, 2000 which is a divisional of U.S. Ser. No. 09/060,048 filed Apr. 14,1998, now a U.S. Pat. No. 6,043,527.

This application is related to the following commonly assignedapplications which are incorporated by reference:

U.S. application Ser. No. 09/028,249 entitled “VERTICAL BIPOLAR READACCESS FOR LOW VOLTAGE MEMORY CELL,” now a U.S. Pat. No. 5,963,469.

U.S. application Ser. No. 08/944,312 entitled “CIRCUIT AND METHOD FOR AFOLDED BIT LINE MEMORY USING TRENCH PLATE CAPACITOR CELLS WITH BODY BIASCONTACTS,” now a U.S. Pat. No. 5,914,511.

U.S. application Ser. No. 08/939,732, entitled “CIRCUIT AND METHOD FORAN OPEN BIT LINE MEMORY CELL WITH A VERTICAL TRANSISTOR AND TRENCH PLATETRENCH CAPACITOR,” now a U.S. Pat. No. 5,907,170.

U.S. application Ser. No. 08/939,742, entitled “CIRCUIT AND METHOD FOR AFOLDED BIT LINE MEMORY CELL WITH VERTICAL TRANSISTOR AND TRENCHCAPACITOR,” now a U.S. Pat. No. 6,066,869, and

U.S. application Ser. No. 08/944,890, entitled “CIRCUIT AND METHOD FORAN OPEN BIT LINE MEMORY CELL WITH A VERTICAL TRANSISTOR AND TRENCH PLATETRENCH CAPACITOR.”

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to integrated circuits and inparticular the present invention relates to integrated circuit memorydevices.

BACKGROUND OF THE INVENTION

Complimentary metal oxide semiconductor field effect transistors (CMOSFETs) are prevalent in integrated circuit technology because theygenerally demand less power than bipolar transistors. Threshold voltagevariations of CMOS transistors, however, are beginning to poseimpractical limitations on CMOS devices as power supply voltages arereduced. In a 0.2 micron CMOS technology a 0.4 V distribution inthreshold voltages might be anticipated. With a one volt power supply,this distribution can cause large variations in the speed of a logiccircuit, such as those used in integrated memory circuits. For example,a threshold voltage of 0.6 V is required in a DRAM memory cell accesstransistor to insure low sub-threshold voltage leakage currents. If athreshold voltage distribution of 0.4 volts is experienced, there willbe instances where little or no excess voltage above threshold voltageis available. As such, data transfer from a memory cell via such atransistor will be very slow.

A basic problem with CMOS access transistors results from the fact thatCMOS devices do not function well at low voltages and require the use ofhigher than desirable power supply voltages, currently around two voltsin 0.2 micron CMOS technology. Various techniques have been proposed tocompensate for this in CMOS technology. For example, some form oftransistor forward body bias, or specialized circuits to compensate forthreshold voltage variations can be used.

Various types of lateral MOS transistors have been described andutilized in CMOS technology. Lateral bipolar transistors have receivedrenewed interest with the advent of bipolar complementary metal oxidesemiconductor (BiCMOS) technologies.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foran access device for use in a low voltage memory device which performsfast read access of memory data.

SUMMARY OF THE INVENTION

The above mentioned problems with integrated circuit memory devices andother problems are addressed by the present invention and which will beunderstood by reading and studying the following specification. A memorycell access device is described which uses a combination of bipolarjunction and CMOS transistors as access devices to store and read dataon a trench plate trench capacitor.

In particular, one embodiment of the present invention provides a memorycell access device that has two access transistors. The first accesstransistor is an n-channel field effect transistor (FET) that is coupledbetween a trench plate trench capacitor and a data communication line.The second access transistor is an NPN bipolar junction transistor thatis coupled between the trench plate trench capacitor and the datacommunication line. The n-channel access transistor and the NPN bipolarjunction transistor are connected in parallel, and a base connection ofthe NPN bipolar junction transistor is coupled to a body of then-channel field effect transistor.

In another embodiment, a low voltage memory cell access devicefabricated as a vertical pillar structure is provided. The memory cellaccess device includes a field effect transistor that is coupled betweena trench plate trench capacitor and a data communication line. Thememory cell access device also includes a bipolar junction transistorthat is coupled between the memory cell and the data communication line.The field effect transistor and the bipolar junction transistor areconnected in parallel, with a base connection of the bipolar junctiontransistor that is coupled to a body of the field effect transistor.

In another embodiment, a memory device having a low voltage supply isprovided. The memory device comprises a plurality of memory cells, aplurality of data communication bit lines, and a plurality of memorycell access devices coupled between the plurality of memory cells andthe plurality of data communication bit lines. Each of the plurality ofmemory cell access devices comprises a field effect transistor and abipolar junction transistor. The field effect transistor and the bipolarjunction transistor are connected in parallel between a trench platetrench capacitor and a data communication bit line.

In another embodiment, a method of accessing a memory cell is provided.The method includes activating a field effect transistor coupled betweena trench plate trench capacitor and a data communication line forwriting data to the memory cell, and activating a bipolar junctiontransistor coupled between the trench plate trench capacitor and a datacommunication line for reading a charge stored on the memory cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of a memory device accordingto the teachings of the present invention.

FIG. 2 is an embodiment of a portion of an array of memory cellsaccording to the teachings of the present invention.

FIG. 3A is a schematic diagram that illustrates an embodiment of amemory cell with a vertical access device according to the teachings ofthe present invention.

FIG. 3B is a cross sectional view that illustrates an integrated circuitembodiment of the vertical access device of FIG. 3A.

FIG. 3C is a cross sectional view that illustrates another integratedcircuit embodiment of the vertical access device of FIG. 3A.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The terms wafer andsubstrate used in the following description include any structure havingan exposed surface with which to form the integrated circuit (IC)structure of the invention. The term substrate is understood to includesemiconductor wafers. The term substrate is also used to refer tosemiconductor structures during processing, and may include other layersthat have been fabricated thereupon. Both wafer and substrate includedoped and undoped semiconductors, epitaxial semiconductor layerssupported by a base semiconductor or insulator, as well as othersemiconductor structures well known to one skilled in the art. The termconductor is understood to include semiconductors, and the terminsulator is defined to include any material that is less electricallyconductive than the materials referred to as conductors. The followingdetailed description is, therefore, not to be taken in a limiting sense.

The term “horizontal” as used in this application is defined as a planeparallel to the conventional plane or surface of a wafer or substrate,regardless of the orientation of the wafer or substrate. The term“vertical” refers to a direction perpendicular to the horizonal asdefined above. Prepositions, such as “on”, “side” (as in “sidewall”),“higher”, “lower”, “over” and “under” are defined with respect to theconventional plane or surface being on the top surface of the wafer orsubstrate, regardless of the orientation of the wafer or substrate.

Smaller integrated circuit devices combined with the development ofvertical integrated circuit structures, make it possible to use bipolarjunction transistor action rather than just field effect transistoroperation. In fact, a bipolar junction transistor structure can be usedas a data read access device and a field effect transistor used as adata write access device, as described herein. Below, an embodiment ofthe present invention is described in terms of a dynamic random accessmemory (DRAM) device. Embodiments of an access device constructedaccording to the teachings of the present invention are also shown anddescribed. It is understood, however, that these embodiments areprovided by way of example and not by way of limitation.

DRAM DEVICE

FIG. 1 is a simplified block diagram of an embodiment of a memory deviceincorporating access devices constructed according to the teachings ofthe present invention. Memory device 100 includes an array of memorycells 102, address decoder 104, row access circuitry 106, column accesscircuitry 108, control circuitry 110, and Input/Output circuit (I/O)112.

In one embodiment, each cell in array 102 includes an access device witha bipolar junction transistor coupled in parallel with a field effecttransistor between a data communication or digit line and a trench platetrench capacitor. The access device is used to charge and discharge thetrench plate trench capacitor to store and read data from the memorycell. The field effect transistor charges the capacitor. The bipolarjunction transistor reads the charge stored on the capacitor bydischarging the capacitor and providing the current to a current senseamplifier. The bipolar junction transistor typically conducts a highercurrent compared to its counterpart field effect transistor. Thus, theaccess device provides the advantage of increased speed in reading thedata stored in the memory cell by allowing the capacitor to bedischarged more quickly.

Memory device 100 can be coupled to an external microprocessor 114, ormemory controller for memory accessing. Memory device 100 receivescontrol signals from the microprocessor 114, such as WE*, RAS* and CAS*signals. Memory device 100 is used to store data which is accessed viaI/O lines. It will be appreciated by those skilled in the art thatadditional circuitry and control signals can be provided, and that thememory device of FIG. 1 has been simplified to help focus on embodimentsof the present invention.

It will be understood that the above description of a DRAM is intendedto provide a general understanding of the memory and is not a completedescription of all the elements and features of a DRAM. Further, theembodiments of the present invention are equally applicable to any sizeand type of memory circuit and are not intended to be limited to theDRAM described above. Other alternative types of devices include SRAM orFlash memories. Additionally, the DRAM could be a synchronous DRAMcommonly referred to as SGRAM, SDRAM, SDRAM II, and DDR SDRAM, as wellas Synchlink or Rambus DRAMs.

Referring to FIG. 2, an embodiment of a portion of array 102 isprovided. The simplified schematic diagram illustrates a portion of acolumn of the array 102. The column is generally defined by a pair ofdata communication or digit lines 120 and 122. Access devices 124 arelocated along the digit lines for coupling trench plate trenchcapacitors 126 to a digit line. Access devices 124 include field effecttransistor 123 in parallel with bipolar junction transistor 125. Thefield effect transistors 123 are activated by a word line (W-WL) towrite data to a trench plate trench capacitor, which defines a row ofthe memory array. The bipolar junction transistors 125 are activated bya word line (R-WL) to read data from a trench plate trench capacitor.

Current sense amplifier 128 is provided to detect current signalsprovided to the digit lines by a bipolar junction transistor during aread operation. In one embodiment, a clamped bit line sense amplifier,as shown, can be used. The current from the digit lines are injected tothe cross coupled pair of inverters (M1/M3 and M2/M4). Transistors M5and M6 serve to clamp the digit lines at a fixed voltage, e.g., 0.7volts. The impedance looking into the source of transistors M1 and M2 isvery low but the current injected here from the data lines serves toupset the cross coupled inverters which provides a high speed largesignal output. Alternatively, other sense amplifiers can be used thatallow a current from a trench plate trench capacitor to be sensed.

As described above, a large variation in a threshold voltage of thefield effect access transistors can result in slow data access. Thisslow access is most troubling in data read operations. Embodiments ofthe present invention avoid this access speed problem while maintaininga higher threshold voltage. That is, for access devices 124 in a DRAMcircuit a larger threshold voltage value is desired to reduce memorycell leakage and increase retention time in the memory cells. By usingthe bipolar junction transistor in parallel with the field effecttransistor, a larger threshold voltage can be maintained withoutincreasing cell leakage and reducing retention time.

Memory Cell Access Device

FIG. 3A illustrates a schematic diagram of a vertical access device 200having both a bipolar junction transistor 202 and a metal-oxidesemiconductor field effect transistor (MOSFET) 204 which can be formedin either bulk or SOI technology. Bipolar junction transistor 202 is anNPN transistor having emitter 206, collector 208 and base 210.Transistor 204 is an n-channel MOSFET having first source/drain region212, second source/drain region 214 and gate 216. The access device canbe fabricated as a single unit, or as separate transistors.

FIGS. 3B and 3C provide alternative fabricated integrated circuitembodiments of an access device 300 constructed according to theteachings of the present invention. FIG. 3B is a bulk silicon embodimentof an access device formed in a single pillar of monocrystallinesemiconductor material. Access device 300 includes a parallelcombination of a bipolar junction transistor and a field effecttransistor. Access device 300 is coupled to trench plate trenchcapacitor 301. Access device 300 includes n+ semiconductor layer 304.Layer 304 serves as an emitter for the bipolar junction transistor, asource/drain region for the field effect transistor and a plate of thetrench plate trench capacitor. P-doped semiconductor layer 306 isfabricated on layer 304. The vertical doping profile of region 306 isvaried, as explained below, to optimize bipolar transistor action. An n+semiconductor layer 312 is provided on top of layer 306. Polysiliconregion 316 is fabricated to operate as a gate isolated from layer 306 bygate oxide layer 314. A polysilicon base contact 320 is providedopposite gate 316 on oxide layer 319 and in contact with region 306.Further, trench plate trench capacitor 301 includes a polysilicon mesh(POLY) that surrounds a portion of layer 304. The polysilicon mesh formsa second plate of capacitor 301.

FIG. 3C is an SOI embodiment of an access device 300 constructedaccording to the teachings of the present invention. In this embodiment,access device 300 includes n+ semiconductor layer 304 that extends downthrough a polysilicon mesh (POLY). The polysilicon mesh and layer 304are formed on insulator layer 303, e.g., an oxide, or insulating baselayer. The remaining components of the access device are substantiallythe same as the access device of FIG. 3B, although fabricationtechniques may differ.

A vertical doping profile of region 306 of the access device isoptimized for both bipolar transistor action and biasing the body of thefield effect transistor to a value around 0.9 V to forward bias the baseemitter junction. The doping profile is controlled so that the topportion 310 of layer 306 is more heavily doped p-type than a bottomregion 308. This difference in doping is represented by the designationsP and P−. The actual doping levels with respect to other structures orbase layers can be varied, and relative doping levels between the topand bottom regions of layer 306 is only represented herein.

One way to create the difference in the doping profile is to use theeffects of the fabrication of emitter 312. When the emitter, or topn-type layer 312, is fabricated a relatively higher base doping levelnear emitter 312 can be created. This doping profile is required in avertical NPN transistor to give field-aided diffusion in the base and ahigh current gain, β. If the base doping is around 10¹⁸/cm³, as iscommon in NPN transistors, then region 310 also serves to make then-channel vertical MOSFET enhancement mode, which is difficult toachieve by other techniques since implantations for threshold voltageadjustment cannot be conveniently done.

The following comparison further illustrates the advantage of using abipolar junction transistor for data read operations in a low voltagememory. If n-channel field effect transistor 204 of FIG. 3A is used todischarge the capacitor, it is customary to precharge a datacommunication “bit” line to ½ V_(DD), or in this illustration 1.5 volts.The peak transfer current I_(D) is estimated to be around 40 μA assuminga Vt of 0.5 volts. That is, the drain current is calculated by:$I_{D} = {\mu \quad {{Co}( \frac{W}{L} )}( {{\frac{( {{Vgs} - {Vt}} )^{2}}{2}\quad {where}\quad W} = {{LI_{D}} = {{80\frac{\mu \quad A}{V^{2}}*1*( {\frac{1}{2}V} )^{2}} = {40\quad \mu \quad A}}}} }$

If the memory cell is assumed to store 50 fC, the charge from the memorycell requires 1.2 nano-seconds to transfer to the bit line throughtransistor 204.

A faster data transfer is possible if bipolar access transistor 202 isused with a clamped bit line where the bit line is precharged to a lowervoltage, such as 0.7 V. The peak bipolar current is determined mostly bythe base current I_(B) and the variation of current gain, β, with peakcurrent. Assuming a base current of 4.0 μA, a peak collector value of400 μA is estimated by:

I _(C) =βI _(B)

where

β=100

If the memory cell is assumed to store the 70 fC, the charge from thememory cell requires only 0.18 nano-seconds to transfer to the bit line.A substantial decrease in transfer time, therefore, is experienced byusing a bipolar access transistor during read operations in the lowvoltage memory.

Access Operations in a DRAM Embodiment

During a write operation the base of BJT transistor 202 is coupled to alow voltage, such as 0.7 volts. The body potential in layer 306,therefore, is held at the low level resulting in a MOSFET body biaswhich increases as the memory is charged due to an elevated bit linepotential. As a result, the threshold voltage of transistor 204 rises toaround one volt. A bootstrapped voltage as known to those skilled in theart can be used to drive the gate voltage above three volts, such asfour volts. This booted voltage is necessary because the supply voltageis limited to three volts, and a second supply is typically notprovided. The time required for the write operation is not critical andcan be much longer than the read response. Thus, the reduced powerrequirements of the MOSFET are desirable.

During a read operation the bit lines are clamped to a low voltage (nearthe base low voltage), in this example 0.7 V. The voltage of the bitlines does not change significantly during a read operation, unlike in amemory using a conventional voltage sense amplifier, since here currentnot voltage is being sensed. During a read operation, the read word linegoes to a higher voltage, such as 1.4 V, to forward bias thebase-emitter junction and turn on the bipolar transistor 202. Thebipolar transistor will be strongly forward biased and quickly dischargethe charge stored on the memory storage capacitor onto the bit linewhere it can be sensed as a current. The memory cell discharges to about0.7 V at which point the bipolar transistor saturates and stopsfunctioning. The memory cell data state voltage levels are thereforeapproximately two volts when charged, and 0.7 V when discharged.

It is estimated that a bit line current sense amplifier is about eighttimes faster than the a bit line differential voltage sense amplifiercommonly used in DRAMs. Further, as detailed above, current transferfrom a memory cell to a bit line using the bipolar transfer device isabout eight times faster than an n-channel MOSFET transfer device. Thenet result is that the present invention, when used in a low voltagememory device for data read operations, is about eight times faster thancommonly used CMOS DRAMs. Further, a vertical access transistor devicewith a trench plate trench capacitor is only 4F² in area. A DRAMaccording to the present invention, therefore, is about one-half thearea of conventional DRAM's and about eight times faster.

In operation, the bipolar device would be used for reads and the MOSFETdevice on the other side of the device pillar can most conveniently beused for write operations to store information on the memory capacitorin a conventional manner. The present invention can be scaled to lowerpower supply voltages and smaller dimensions, in which case the use ofthe bipolar access device becomes yet more advantageous. For one voltpower supply voltages, the threshold voltage variations of MOSFETs willbecome a large fraction of the total voltage available.

Conclusion

Embodiments of an access device for a memory device have been describedwhich use an n-channel field effect transistor and a bipolar junctiontransistor coupled in parallel between a trench plate trench capacitorand a data communication line. A base connection of the NPN bipolarjunction transistor has been described as coupled to a body of then-channel access transistor to control threshold voltage variations ofthe n-channel field effect transistor. During operation the n-channelfield effect transistor is used for writing data to a trench platetrench capacitor, while the NPN bipolar junction transistor is used forread operations in conjunction with a current sense amplifier circuit.The access transistors are described as fabricated as a single verticalpillar.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention.

What is claimed is:
 1. A method of reading data from a memory devicecomprising: selectively accessing at least one of a plurality of memorycells with a vertical bipolar junction transistor; and detecting currentsignals from the vertical bipolar junction transistor using a currentsense amplifier.
 2. The method of claim 1, and further comprisingclamping a bit line at a first voltage while reading data.
 3. The methodof claim 2, wherein clamping the bit line comprises clamping the bitline at approximately 0.7 volts.
 4. The method of claim 1, whereinselectively accessing at least one of a plurality of memory cellscomprises raising a base voltage of the bipolar junction transistor to afirst selected voltage.
 5. The method of claim 4, wherein raising thebase voltage comprises raising the base voltage to approximately 1.4volts.
 6. The method of claim 1, wherein detecting current signals fromthe vertical bipolar junction transistor comprises detecting currentsignals from the memory cell having a data state voltage level ofapproximately two volts.
 7. The method of claim 1, wherein detectingcurrent signals from the vertical bipolar junction transistor comprisesdetecting current signals from the memory cell having a data statevoltage level of approximately 0.7 volts.
 8. A method of reading datafrom a memory device comprising: selectively accessing at least one of aplurality of memory cells with a vertical bipolar junction transistor byapplying a first voltage to a read word line coupled to the base of thevertical bipolar junction transistor; and detecting current signals fromthe vertical bipolar junction transistor using a current senseamplifier.
 9. The method of claim 8, and further comprising clamping abit line at a second voltage while reading data.
 10. The method of claim9, wherein clamping the bit line comprises clamping the bit line atapproximately 0.7 volts.
 11. The method of claim 8, wherein applying thefirst voltage to a read word line comprises raising the first voltage toa read word line to approximately 1.4 volts.
 12. The method of claim 8,wherein detecting current signals from the vertical bipolar junctiontransistor comprises detecting current signals from the memory cellhaving a data state voltage of approximately two volts.
 13. The methodof claim 8, wherein detecting current signals from the vertical bipolarjunction transistor comprises detecting current signals from the memorycell having a data state voltage level of approximately 0.7 volts.
 14. Amethod of reading data from a memory device comprising: selectivelyaccessing at least one of a plurality of memory cells with a verticalbipolar junction transistor by applying a first voltage to a read wordline coupled to the base of the vertical bipolar junction transistor;maintaining a second voltage on a write word line coupled to a gate ofan n-channel field effect transistor having a body coupled to the baseof the vertical bipolar junction such that the n-channel field effecttransistor is off; and detecting current signals from the verticalbipolar junction using a current sense amplifier.
 15. The method ofclaim 14, and further comprising clamping a bit line at a third voltagewhile reading data.
 16. The method of claim 15, wherein clamping the bitline comprises clamping the bit line at approximately 0.7 volts.
 17. Themethod of claim 14, wherein applying a first voltage to a read word linecomprises raising the voltage to a read word line to approximately 1.4volts.
 18. The method of claim 14, wherein detecting current signalsfrom the vertical bipolar junction transistor comprises detectingcurrent signals from the memory cells having a data state voltage levelof approximately two volts.
 19. The method of claim 14, whereindetecting current signals from the vertical bipolar junction transistorcomprises detecting current signals from the memory cell having a datastate voltage level of approximately 0.7 volts.
 20. A method of readingdata from a memory device comprising: selectively accessing at least oneof a plurality of memory cells with a vertical bipolar junctiontransistor; detecting current signals from the vertical bipolar junctiontransistor using a current sense amplifier; and operating the verticalbipolar junction transistor using a one volt power supply voltage. 21.The method of claim 20, wherein an emitter base junction of the verticalbipolar junction transistor is forward biased.
 22. A method of readingdata from a memory device comprising: selectively accessing at least oneof a plurality of memory cells with a vertical bipolar junctiontransistors; clamping a bit line at a selected voltage while readingdata; detecting current signals from the vertical bipolar junctiontransistor using a current sense amplifier; and operating the verticalbipolar junction transistor using a one volt power supply voltage. 23.The method of claim 22, wherein detecting current signals from thevertical bipolar junction transistor comprises detecting current signalsfrom the memory cell having a data state voltage level approximatelyequal to the voltage at which the bit line is clamped.
 24. A method ofreading data from a memory device comprising: selectively accessing atleast one of a plurality of memory cells with a vertical bipolarjunction transistor by applying a first voltage to a read word linecoupled to the base of the vertical bipolar junction transistor;maintaining a second voltage on a write word line coupled to a gate ofan n-channel field effect transistor having a body coupled to the baseof the vertical bipolar junction transistor such that the n-channelfield effect transistor is off; detecting current signals from thevertical bipolar junction transistor using a current sense amplifier;and operating the n-channel field effect transistor and the verticalbipolar junction transistor using a one volt power supply voltage. 25.The method of claim 24, wherein an emitter base junction of the verticalbipolar junction transistor is forward biased.
 26. A method of readingdata from a memory device comprising: selectively accessing at least oneof a plurality of memory cells with a vertical bipolar junctiontransistor by applying a first voltage to a read word line couple to thebase of the vertical bipolar junction transistor; maintaining a secondvoltage on a write word line coupled to a gate of an n-channel fieldeffect transistor having a body coupled to the base of the verticalbipolar junction such that the n-channel field effect transistor is off;clamping a bit line at a third voltage while reading data; detectingcurrent signals from the vertical bipolar junction transistor using acurrent sense amplifier; and operating the n-channel field effecttransistor and the vertical bipolar junction transistor using a one voltpower supply voltage.
 27. The method of claim 26, wherein detectingcurrent signals from the vertical bipolar junction transistor comprisesdetecting current signals from the memory cell having a data statevoltage level approximately equal to the third voltage at which the bitline is clamped.